Hydraulic operating systems for pipeline valves are known in the art. In such systems, a closed hydraulic circuit provides pressure to a piston which operates against a spring. Upon an emergency situation, such as a pipeline leak or break, the pressure against the spring is released and the spring closes a valve in the pipeline until the emergency situation can be corrected at which time the system is re-pressurized to open the valve.
One of the major problems with such systems is that when in their natural environment, they can be exposed to a wide range of ambient temperature fluctuations, not only winter to summer temperature variations but also the variations which might occur from nighttime to daytime. For example, if colder situations are encountered after the system is initially set, contraction of the hydraulic fluid and the concomitant reduction in pressure on the operating spring could cause an undesirable valve drifting situation. Similarly, as the ambient temperature heats up, the expansion of the fluid in the closed system puts undue pressure on the system components which may result in their failure.
In an attempt to at least partially solve this problem, one prior art system is provided with a relief valve such that upon warmer ambient conditions, the expanded hydraulic fluid blows by the relief valve so that undue pressure is not placed on the operating spring. However, when the ambient temperature later cools, the drop in hydraulic pressure in the system allows the spring to move the operating piston potentially resulting in a partial closing of the pipeline valve.
In another similar system a hydraulic accumulator is provided to receive the excess hydraulic fluid upon thermal expansion. However, not only does such an accumulator significantly add to the expense of the system, but also it is plagued with the same problems encountered by the system with the relief valve just described, that is, when the ambient temperature cools, the hydraulic fluid in the accumulator cannot return to the actuator.
Thus, despite the need for a system which can account for both thermal expansion and thermal compression, no one has developed such a system, let alone a system which does not require additional operating components.